The preparation of surface relief structures in photosensitive materials is well known. Over the years, the art has learned how to reduce the dimensions of the relief lines or holes (viewed normal to the surface) to the point where they are measured in terms of micrometers and fractions of micrometers.
Extensive studies have been made, in particular, of periodic one dimensional structures prepared in positive type photosensitive materials or photoresists. (It will be appreciated that the terms "one dimensional" and "two dimensional" are terms of art describing the relief structure from a view normal to the surface). One type of photoresist contains a photosensitive polymer which when exposed to light becomes soluble in an appropriate water base developer. After exposure and development, the initially flat surface of the photoresist becomes a surface relief structure whose depth varies depending upon the photoresist that has been etched away by the developer in proportion to the exposure light intensity. When exposed to an intensity variation that is periodic, such as a light interference pattern, a periodic surface profile will be formed which is everywhere proportional to the initial interference intensity pattern. Precise relief structures of this type can be made easily over relatively large areas using laser interference techniques.
It is known in the art to form relief diffraction gratings employing laser interference techniques. In general, such gratings are formed by exposing a photosensitive material such as a photoresist to two coherent interfering laser beams (recording beams) whose wave fronts are substantially plane and parallel. When such beams interfere, there is produced a stationary periodic fringe pattern consisting of maxima and minima of intensity. The spacing between adjacent maxima (or minima) is determined by the angle between the beams and by the wavelength of the exposing light. Depending upon the optical system used, substantially any spacing can be obtained down to about half the wavelength of the exposing light. The photosensitive material will thus be exposed to a periodic variation in intensity across its surface.
The above description applies to the formation of straight line gratings (one-dimensional gratings); that is, the maxima or minima of the developed image appear as straight parallel lines when viewed normal to the surface. A crossed grating (two-dimensional grating) can be obtained by rotating the photosensitive material 90.degree. about an axis perpendicular to the center of the surface subsequent to the first exposure and exposing a second time. In this case, the surface is subjected to two periodic intensity variations at right angles to each other. Upon development, the resulting relief structure will consist of a rectangular array of peaks and valleys; in the case of a positive photoresist, the peaks correspond to the areas where the combined intensity of the two exposures was the least, or where there was no exposure, and the valleys to the areas where the exposure was the greatest.
Variations in the symmetry of the above described array are also possible. For example, by changing the angle between the two beams after the first exposure, one obtains a different grating spacing for each of the two perpendicular orientations. This can lead to points of intersection which are oblong instead of round. Alternatively, if the exposure plate is rotated to form an angle other than 90.degree. between the two exposure positions, one would obtain a diamond-shaped rather than a square point of intersection array.
If such photosensitive material is a positive photoresist, then upon application of a developer those areas receiving the largest exposure will be preferentially etched away relative to those areas receiving the least exposure. After sufficient development time, the photoresist surface will be a periodic relief pattern whose depth depends on the original interference exposure. A positive photoresist is rendered soluble by impinging light and thereby susceptible to etching by the developer. Alternatively, a photoresist could be chosen which would harden upon photoexposure (negative photoresist) whereupon the unexposed areas would be dissolved by appropriate treatment.
As examples of positive-acting photoresists mention may be made of initially hydrophobic, low molecular weight resins containing sensitizer, which, upon absorbing radiation change the solubility of the coating from aqueous alkali insoluble to aqueous alkali soluble. Suitable resins include phenol formaldehyde novolaks, novolaks in combination with styrene, methyl styrene and styrene-maleic anhydride copolymers and melamines.
As examples of representative negative-acting photoresists, mention may be made of polyvinyl cinnamate derivatives, vinyl ester containing cinnamylidene and alkyl ester prepolymers.
Additional details regarding positive and negative photoresist may be found, for example in W. S. DeForest, Photoresist Materials and Processes, McGraw-Hill, N.Y., 1975.
The photoresist is applied to any suitable substrate such as glass, silicon, plastic film or the like, through conventional precedures. Positive photoresists are preferred and they are usually applied in liquid form at room temperature to an appropriately cleaned substrate by spin coating in thicknesses which range from a fraction of one micron to a few microns depending on the spin rate and the photoresist. Dip coating and other coating techniques can also be utilized. In order to drive off any remaining solvents, the photoresist layer and substrate are usually exposed to an elevated temperature for a short period of time, a procedure known as "pre-baking", typically 90.degree. C. for 20 minutes. Sensitivity of the photoresist is usually greatest without any pre-baking and a long pre-bake at lower temperature, e.g., 1 hour at 70.degree. C., leaves the photoresist more sensitive than a short pre-bake at elevated temperature, e.g., 30 minutes at 90.degree. C.
The photoresist is, of course, exposed to light to which it is sensitive in a predetermined pattern and then developed. Typical lasers and their associated wavelengths include the argon ion laser (458 nm) and the He-Cd laser (442 nm) with coherence lengths of about 5 cm and 12 cm, respectively. As shown in FIG. 1, light from the laser, polarized perpendicular to the plane of incidence, is split by a 50-50 beam splitter so that half of the light intensity is incident on one mirror and the other half is incident on a second mirror. The light is reflected from each of these mirrors through an expansion lens and a spatial filter such that the expanded beam is incident on the whole target area where the photoresist coated substrate is located. The two intersecting beams, being derived from one coherent source and having traveled along substantially equal paths from the beam splitter, form interference fringes. The spacing (d) between adjacent maxima (or minima) is given by the equation d=.lambda./(2 sin .theta.) in which .lambda. is the wavelength of the laser light and .theta. is the angle of incidence to the target plate of each beam (i.e., one-half of the angle between the two beams).
Exposure is carried out until a sufficient intensity level is attained. Exposure times are related to the size of the target area to be exposed. For example, for the He-Cd laser (.lambda.=442 nm) with a spatial filter distance to the target of 50 inches, a 40 times expansion lens and a square target 10 inches on each side, exposure times of up to one hour are necessary. With the argon laser, the exposure times are approximately half as large, since even though the available power is larger, the photoresist is less sensitive at the longer wavelength.
Knop, in U.S. Pat. No. 3,957,354, describes a subtraction filtering techniques which employs diffraction for discriminatorily subtracting unwanted spectral wavelength portions of polychromatic illuminating light. The zero diffraction order color characteristics of the subtractive color filter employing a diffracting medium, including a pattern of spatially distributed diffraction elements, are determined solely by the waveform profile of each diffraction element and the absolute magnitude of the effective optical peak amplitude of the waveform profile.
Fletcher et al., U.S. Pat. No. 3,815,969, teaches a holographic recording medium employing a substrate having a diffraction grating composed of a plurality of spaced line ridges on a surface thereof.
Linear relief patterns formed by laser beam interference have been used to produce crystals in a specific crystallographic orientation. Thus, "Orientation of Crystalline Overlayers on Amorphous Substrates by Artificially Produced Surface Relief Structure" by Dale Clifton Flanders, submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy, Massachusetts Institute of Techniology, Cambridge, Mass., January 1978 describes linear relief structures and the growing of single potassium chloride crystals thereon. The crystals assumed the orientation of the substrate and were randomly located along the troughs of the relief pattern and non-uniform in size. The linear relief structure in the substrate was produced by soft X-ray lithographic exposure through a mask produced by laser beam interference followed by reactive-ion-etching. Similar procedures are described in Appl. Phys. Lett., Vol. 32, No. 6, Mar. 15, 1978, p. 349-350.
In Scanning Electron Microscopy, 1978, Vol. 1, SEM Inc., A.M.F., O'Hare, Ill., p. 33-40, there is set forth a procedure for fabricating gratings having linewidths of 100 nm and less wherein a photoresist is exposed to radiation in some desired pattern. It is stated that, "The radiation can be a scanned beam, a focused optical or electron image, a hologram, or an optical or X-ray shadow of a mask. Following exposure, a development step removes either be exposed or unexposed regions (i.e., positive or negative resist), thereby leaving a resist pattern in relief on the substrate surface."
Horst et al., U.S. Pat. No. 4,404,939, teaches the preparation of a diffraction grating master which can contain up to 800 lines per millimeter, in which two series of diffraction gratings are exposed on a photoresist which is thereafter developed. Although the two series of lines can be exposed on the photoresist before developing, the patent states that the superimposed diffraction gratings in the photoresist coating are more sharply defined when the photoresist is developed after each exposure to a grating mask.
Gale, Sinusoidal Relief Gratings For Zero-Order Reconstruction Of Black-And-White Images, Optical Communications, Vol. 18, No. 3, p. 292 (1976) teaches that high quality black-and-white images can be reconstructed using zero order transmitted light from surface relief sinusoidal phase gratings modulated with image information. If the grating period is chosen sufficiently fine, these recordings can be read out in conventional slide projectors and microfiche readers. The article discloses the formation of a crossed, 1.4 .mu.m grating pattern by exposing a photoresist to an interference pattern using a laser and then rotating the substrate 90.degree. and re-exposing the substrate.
It has long been known that to achieve a given visual result (color, light intensity, etc.), the profile of the surface relief structure should have a known particular configuration. In order to realize such relief structures, the art has employed various techniques to structure or manipulate the light pattern to which the photoresist will be exposed since the photoresist will form a positive or negative image (depending on whether a positive or negative photoresist is used) of that pattern. As the elements of the relief pattern have become smaller, i.e., as the number of lines per millimeter has increased, it has become more and more difficult to prepare intricate patterns to be projected onto or exposed on a photoresist. This is particularly true where the width of a line or diameter of a hole in the photoresist in one micron or less, i.e., a pattern having 1,000 or more lines per millimeter.
A method has now been discovered in which accurate relief patterns can be made in a photoresist in which the individual features of the relief pattern can be of submicron size and in which the size and shape of the relief pattern is controlled, to a great extent, by manipulation of the exposure and development parameters of the photoresist as distinguished from the pattern or image to be projected onto the photoresist. The invention permits an almost infinite variety of predetermined contours to be realized, ranging from tiny holes in a flat surface to projections rising from a surface in the form of flat-topped pedestals or steep spires or lollipop shaped projections, with or without channels, the longitudinal walls being terraced or unterraced as desired. Diffraction gratings of any desired geometric configuration, symmetrical or asymmetrical, can be achieved. The individual holes in the photoresist surface can be circular, oblong or diamond shaped, as desired.
It is, accordingly, the object of this invention to provide a method in which relief contours in a photosensitive material can be fabricated to provide any predetermined desired shape or geometric configuration. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.